Cochlear implants (“CI”s) may restore the ability to hear to deaf or partially deaf individuals by providing electrical stimulation to the auditory nerve via a series of electrodes placed in the cochlea. Sound input activates the electrodes of the CI with different frequency bands being assigned to the various electrodes based on the tonotopic organization of the inner ear. The placement of each electrode within the cochlea is related to the range and value of each frequency band, with electrodes closer to the base of the cochlea generally corresponding to higher frequency bands. CIs may successfully provide the ability of almost all postlingually deaf users (i.e., those who lost their hearing after learning speech and language) to gain an auditory understanding of an environment and/or restore hearing to a level suitable for an individual to understand speech without the aid of lipreading.
After the CI is put in place, sound is picked up by a microphone and sent via a speech processor of the CI to the electrodes. After implantation, it often takes an audiologist several months of fine-tuning (and it takes the patient several months of experience with the CI) before the full efficacy of the CI is reached. Even after programming has been completed, the distortion of auditory input associated with CI's (e.g., spectral degradation and frequency shift) often requires extensive perceptual learning on the part of the patient. Spectral degradation in the CI is caused by the limited number of stimulation channels while frequency shift results from physical limitations on the electrode insertion depth which may cause a mismatch between the speech processor's analysis filters and the characteristic frequency of the neurons stimulated by the electrodes.
Stimulation parameters for the CI include a frequency map which determines the electrodes to be stimulated in response to sound of a given frequency. Most patients undergoing electrical auditory stimulation are presented with a standard frequency map.